Visual simulation for purposes of training is well-known in the art. For example, such simulation systems can take the form of a cathode ray tube which is projected against a half-silvered mirror in front of the windscreen of a simulated aircraft cabin through an optical system which collimates the rays of light leaving the television screen, thereby making them appear to originate at a point very far from the observer. Such collimation is important in view of the fact that a directly viewed television picture will not appear realistic if the observer moves his head. In the case of head movement, the perceived image presented by such a television screen will appear to move. On the other hand, if collimation is used, the object will not appear to move any substantial distance and thus the impression of an object at a great distance will be created.
As an alternative, an actual model of the scene outside the windscreen of an aircraft can be placed outside the aircraft. Here, also, collimating optics must be used to create the proper sense of distance. However, an additional variable is introduced when such a model is used. In particular, the model's movements result in a change of range with respect to the observer. Accordingly, the optical system must be adjusted in order to continue acting as an effective collimator to scale up the apparent range.
The problems with respect to collimating rays of light from models generally stem from the fact that a collimating system, such as a concave mirror will only collimate light originating in its focal plane. Accordingly, if a model is moved substantially away from the focal plane of the collimating optics, the rays of light, instead of being parallel will converge or diverge depending upon whether the model appears before or after the focal plane of the collimating optical system. A solution to this problem is suggested in U.S. Pat. No. 3,190,171 of G. A. Reed. In this patent, the problem of collimating an object at different positions is solved by moving large focusing mirrors through which the object is viewed. In particular, Reed suggests moving the mirror directly in front of the observer. Apart from design problems which arise as a result of providing a realistic position for the windscreen and sufficient space for the mirror in front of the windscreen to move, such a structure poses the additional problem of alignment and the difficulty of moving large optical elements. If, in addition, it is desired to provide a simulator having dislays for a pilot and co-pilot, these problems are multiplied.